Priority is claimed to Japanese Application No. 10-360881, filed Dec. 18, 1998, herein incorporated by reference.
1. Field of the Invention
The present invention relates to a projection image display device for displaying a projection image on the pupil of an observer using a reflective type display element, and particularly relates to displays suitable for head-wearable type projection image display devices, i.e., so-called head-mounted displays, which require a high resolution projection image display in a compact form.
2. Description of the Related Art
Projection image display devices, which are worn on the head of an observer, project a virtual image of an image generated by a projection image forming member using a viewing optical system and direct the virtual image to the pupil of the observer. Generally, known projection image display devices are conventionally referred to as head-mounted displays (HMD). The liquid crystals used for the projection image forming member may be broadly divided into transmission-type liquid crystals illuminated from the opposite side from the display side, and reflective-type liquid crystals illuminated from the display side.
The transmission-type liquid crystal display panel widely used in projection image display devices, requires a high transmittance member such as glass or the like as a substrate to transmit illumination light from the back side. High density pixel integration is difficult due to the substantial limitations of the semiconductor manufacturing process used to make these devices. Furthermore, a peripheral circuit area of a certain size such as a Thin Film Transistor (TFT) is required to drive each pixel, and since the integration of the pixels must occur on the same display panel, the pixel area of the whole display panel, i.e., aperture efficiency, is reduced so as to result in disadvantageous reduction of illumination efficiency and image quality.
On the other hand, the reflective-type liquid crystal display panel uses a semiconductor such as silicon as a substrate to provide a compact and high integration display, and since the circuit area can be arranged on the opposite side relative to the display side there is only slight reduction in aperture efficiency even when the integration is increased, so as to provide very high reflectivity to produce a bright image. Since the size of the pixels is very small, using the same number of pixels the size of the display panel can be reduced without decreasing brightness. In a similar size panel the number of pixels can be increased to increase the resolution. Furthermore, the display switching speed is improved since it is theoretically possible to reduce the thickness of the liquid crystal layer for polarization conversion.
If a reflective-type liquid crystal display panel having these many advantages is used in a head-mounted display, a bright high-resolution image can be largely attained in a head-mounted display. FIG. 9 schematically shows the construction of a conventional projection image display device using such a reflective-type liquid crystal display panel. A reflective-type liquid crystal display panel 51 is used as a projection image display element, and a polarization beam splitter (PBS) 52 separates the viewing light reflected by the reflective type liquid crystal display panel 51 and the illumination light from the illumination light source 53, and the viewing optical system eyepiece lens 54 directs the light to the pupil of the user 55, which sees an enlarged projection image.
FIG. 8 schematically shows the illumination light source 53 disposed near the reflective type liquid crystal display panel 51 and a diffusion panel 56 arranged on the front surface of the illumination light source 53 to diffuse the illumination light from the light source entering the reflective type liquid crystal display panel 51, and the reflected viewing light is directed by the eyepiece lens 54 to the pupil 55, which sees an enlarged projection image.
When using the reflective type liquid crystal display panel, the illumination light must illuminate from the display side of the panel, and the light source must be arranged so as to avoid interfering with image viewing, thereby making it difficult to achieve uniform illumination of the entire surface of the display panel. The construction shown in FIG. 9 does not have an illumination optical system possessing optical power, only the illumination light and the viewing light are separated, there is no conjugate relationship between the illumination surface 53a and the pupil 55, and a large illumination surface is required. When the size of the illumination light source 53 is reduced under such conditions, the light flux on the fringes of the field of view is diminished such that the field of view has a dark fringe.
The construction shown in FIG. 8 provides a diffusion panel on the front surface of the light source to improve the uniformity of the illumination on the display panel, but the result of adequately diffusing the illumination light from the light source is a reduction in illumination efficiency.
In view of the previously described disadvantages, an object of the present invention is to provide a compact and high resolution projection image display device using a reflective-type display element which uniformly illuminates the entirety of the display element without reducing illumination efficiency, and ensures viewing conditions of excellent and uniform brightness to the fringe of the field of view.